The DSM-IV definition of Attention-deficit Hyperactivity Disorder (ADHD) distinguishes a new subtype of ADHD not identified in DSM-III or DSM-III- R. Children who meet criteria for the Predominantly Hyperactive-impulsive Type (PreH) exhibit high levels of hyperactivity and impulsivity, but are not reported to have the deficits in attention considered to be the hallmark of ADHD. This subtype was distinguished on developmental grounds: Because three-quarters of children with the PreH Type are < 7 years of age, it was assumed that they represent milder cases of the full ADHD syndrome in young children whose attention deficits are not yet evident. As children with this subtype of ADHD grow older, it is expected that they will be reported to exhibit higher levels of inattention as a result of being placed in more challenging educational environments that reveal their mild, but still impairing attention deficits. Therefore, the new PreH subtype may facilitate the early identification of ADHD at preschool ages when inattention cannot be measured well. It is very important to test this developmental model, however, as there are important reasons for concern that the PreH Type may actually lead to the incorrect identification of normal, active children who do not have attention deficits, are not functionally impaired, and who will soon outgrow their motor hyperactivity. Unlike children of the same age with the full ADHD syndrome, the PreH Type did not differ from controls on any measure of impairment in peer social relationships or preschool adjustment in the DSM-IV Field Trials (they were only rated as impaired by adults on global measures). Therefore, the validity of the PreH Type in preschool children will be examined in the proposed study by testing the hypothesis that they will exhibit significantly less attending behavior than matched control subjects in challenge situations that will reveal their attention deficits, and will exhibit greater impairment than controls on measures of social competence and mother-child interaction that are more sensitive and age-appropriate than those used in the DSM-IV Field Trials. The validity of the PreH Type also will be examined developmentally by testing the hypothesis that they will be reported to exhibit progressively more inattention symptoms and academic impairment over four years as a result of increasingly more challenging educational demands. Children with the full syndrome of inattention and hyperactivity-impulsivity (DSM-IV Combined Type) are expected to show the same forms of impairment as the PreH Type, but with greater severity at the same ages. In addition, the proposed study would be the first study of the stability of the diagnosis of ADHD over time in preschool children and the first prospective examination of the early emergence of serious conduct problems in young children with ADHD.